New York (AP) -- Nearly everything is going according to the plan Federal Reserve Chairman Ben Bernanke hatched six months ago.

During a speech in Jackson Hole, Wyo., on Aug. 27, Bernanke outlined an effort to spur economic growth, prevent prices from falling and push markets higher through the purchase of government bonds. Since then stocks have soared, the unemployment rate has dropped and Americans have started to spend more.

"It's been a success," says Bill Gross, who manages the world's largest mutual fund at Pimco. Gross had skewered Bernanke's attempt to boost the economy, comparing it to a Ponzi scheme. "It's hard to dispute that since Jackson Hole the market is up around 25 percent."

But the Fed's $600 billion program to buy Treasurys ends in June. And investors like Gross are worried the stock and bond markets will fall without the Fed's $75 billion monthly injection. "At the end of June, the biggest bond buyer steps away," he says. "The markets could have a shock in store."

And now there's a different economic issue. Higher prices for food and energy have replaced a double-dip recession as the major concern for economists and investors. Bernanke begins two days of Congressional testimony Tuesday and is sure to face criticism that the bond-buying program known as quantitative easing is to blame.

On the surface, Bernanke's speech in Jackson Hole was full of Fed-speak. But the language was clear to those in the audience at the Fed's annual Board of Governors meeting in the Wyoming resort. "He was saying, 'Whatever it takes we're going to do,'" says Richard Hoey, chief economist at BNY Mellon. "It was a Rambo message."

The move, which began in November, was unorthodox, but the logic was simple: Buying $600 billion in Treasurys would make borrowing cheaper and move investors out of low-yielding bonds into riskier investments like stocks. A rising stock market could then give Americans confidence in the economy and spur consumer spending, which leads to higher corporate profits.

A lot has happened in the markets and the economy since then - most of it good.

-The unemployment rate dropped to 9 percent in January, the most recent month for which data is available. It was 9.6 percent in August.

-The Consumer Price Index rose 0.4 percent in January and 1.6 percent over the previous year. Prices rose 0.2 percent last August from the month before and just 1.1 percent over the previous year.

-The Standard & Poor's 500 stock index is up 27 percent since Aug. 26, the day before Bernanke's speech, powered by stronger corporate profits and people moving their savings into stock funds.

-Consumer spending has climbed seven months in a row. In the last quarter of 2010, it grew at the fastest pace in three years. Spending rose 0.2 percent in January, according to data released Monday.

"Measured in the fairest possible way, and by just about every measure, QE2 has succeeded so far," says Anthony Chan, chief economist at JPMorgan's wealth management unit. QE2 is market slang for the Fed's quantitative easing program.

Long-term interest rates are the exception. They've been on a steady climb, until the recent turmoil in Egypt and Libya pulled them lower. The benchmark 10-year Treasury rate recently hit 3.50 percent. That's up from 2.49 percent on Aug. 26. But rates fell after the Jackson Hole speech and then began rising on each bit of good news about the economy. Economists say that's how it's supposed to work. Interest rates typically rise during an economic recovery to compensate bond holders for the negative hit from inflation.

It's another story if rates jump too quickly. "Obviously, higher rates can stop an economy dead in its tracks," he says.

How could rising rates derail the Fed's efforts? A common worry among investors is that the Fed proves too successful in pushing up prices and inflation gets out of control, leading to a spike in rates.

"It has brought us closer to an unstable rise of inflation expectations," he says. "There's a risk of exceptional instability."

Another danger: There will be nobody to replace the Fed when the program ends in June. "Who will buy when the fed stops buying?" Gross asks, rhetorically. "Who will take their place? Mutual funds like Pimco?"

That's unlikely, he says, because investors have been pulling money out of bond funds. Other institutional investors, like insurances companies and banks, are beginning to put their money elsewhere. And even bond managers like Gross have been warning investors away from Treasurys for months. If rates rise too high and too quickly, they squelch the economic recovery and drive down stocks.

Jack Albin, chief investment officer at Harris Private Bank, worries that the U.S. could wind up with a similar experience to Japan's. The Japanese central bank also managed to boost prices and spur economic growth through pushing money into financial markets starting in 2001. Bond yields began rising. But the benefits evaporated when the central bank pulled back.

"Maybe it was a problem of timing," he says. "But once they took the quantitative easing programs off, the economy just sagged back to where it was before."